This invention pertains, in general, to electromagnetic transducers, and more particularly to transducers for converting electrical energy into movements or sounds, and/or movements or sounds into electrical energy.
Electromagnetic transducers for converting movements or forces (such as sound) into electrical energy, or for converting electrical energy into movements or forces (such as sound), include an electrically conductive moveable element (armature) and a stationary magnetic element (stator) for applying magnetic flux thereto. Forces applied to the armature, as a result of magnetic flux variations, convert electrical energy into movements, and in the case of a speaker, sound. Alternately, physical forces on the armature (such as in the case of microphones) create changes in the magnetic flux and generate electrical signals due to the movement of the armature.
In the prior art speakers, the armature is located in a magnetic field and is generally secured near the center of the diaphragm (speaker cone). The armature usually includes a coil that receives the electrical signals to be converted into sound. The high frequencies are generated at the center of the diaphragm while the intermediate and base frequencies are generated over the entire surface of the cone. Such an arrangement has poor efficiency at high frequencies and as a result usually has a degraded high frequency response. In order to obtain an improved high frequency response, a small "tweeter" type speaker cone has been mounted within the center of the large speaker cone to get greater power output at high frequencies. Other arrangements include separate base and tweeter speakers electrically interconnected with crossover networks to provide an overall high fidelity response. In either case, great care must be taken in the design and manufacture thereof so that the frequency response of the combination provides a continuous transition between high and low frequency ranges, or else certain frequencies will be attenuated, or peaked, resulting in an undesirable frequency response.
The present day speakers use a magnetic structure that is cup shaped, with an inner pole piece extending through the center of the opening. The open edge of the cup forms one magnetic pole while the free end of the inner pole piece forms the other magnetic pole. The air gap between the pole pieces is reduced to increase the concentration of magnetic flux. In order to achieve greater magnetic flux density and higher power output in the high frequency range, the permanent magnets used in the magnetic assembly of the prior art are increased in size so that the speakers are very often rated in "pounds" of magnet (i.e., 2 pound magnet speaker, etc.). This increased size and weight add further structural requirements to the speaker housing so that the speaker will not warp. These requirements add to the cost of the speaker. In addition there is a point of diminishing returns wherein added incremental sizes of magnets will produce diminishing amounts of additional power output at the high frequency range. Furthermore, since it is the strength of the magnetic flux density through the armature that determines the efficiency and the output of the speaker, the size of the magnet (in pounds) may be misleading in cases where poor magnetic flux coupling is provided as a result of poor design.
In order to reduce the size and weight of the magnetic structure of speakers, various peripherially driven transducers were designed, such as for example, those disclosed in the U.S. Pat. Nos. 2,520,646 and 2,535,757 issued on Aug. 29, 1950 and Dec. 26, 1950, respectively, to E. E. Moth and J. J. Root, respectively. These transducers include armature plates secured to the periphery of the diaphragm and a combined permanent magnet and electromagnetic stator arrangement to impart varying magnetic flux to the armature. Such an arrangement provides sufficient power and fidelity for telephone purposes as set forth in U.S. Pat. No. 2,520,646. The arrangement disclosed in U.S. Pat. No. 2,535,757 may have provided sufficient fidelity over 20 years ago, but would probably be considered poor in today's standards. The limited fidelity results from the inherent disadvantages in driving a speaker by modulating the magnetic flux of the stator through the use of a stator coil. The coil required to provide the necessary flux variation introduces a large reactive impedance in the circuit that limits its overall frequency response. It was found that greater efficiency and higher frequency responses would be achieved with the coil mounted on the movable armature as in most present day speakers.
Peripherially driven electroacoustic transducers have been developed wherein the cup shaped magnetic speaker structure of the prior art is used along with a coil attached to the peripheral edge of a diaphragm extending into the air gap. Because of the massive magnetic structure involved (wherein essentially the entire central portion of the cup shaped magnet is formed speaker iron), the arrangement, as a practical matter, is limited in size, for use as a microphone, or as a tweeter speaker. Any increase in size to provide a good base frequency response would be prohibitive because of weight and cost.
Electromagnetic transducers are presently being used in control apparatus, such as a pneumatic controllers (for converting electrical signals, into pressure, or pressure into electrical signals) that employ the same cup shaped magnetic structure as presently used in speakers to drive a ring shaped armature. Because of the weight and size of such a magnetic structure, and the lack of available space, the forces, or electrical signals, produced by such transducers are rather limited. As a result, intricate lever mechanisms must be used with such transducers to provide the desired conversion and hence add to the expense of such apparatus. It would be highly desirable if a transducer could produce the forces, or electrical signals, needed for such control apparatus without requiring the massive and heavy magnetic structure, particularly so if greater forces or higher amplitude electrical signals, could be produced without requiring additional space.
In addition to the foregoing, the electroacoustic transducers of the prior art are highly directional. Most of the acoustic energy generated by the electroacoustic devices is directed in a cone shaped pattern from the center of the diaphragm. If an omidirectional speaker arrangement is desired, a large number of speakers are required to be mounted in some sort of directional array so that the combined effect of all the speakers provides the desired effect. Such an arrangement is quite expensive, requiring many speakers.
It is therefore an object of this invention to provide a new and improved electromagnetic transducer for converting electrical energy into movement, and/or converting movement into electrical energy.
It is also an object of this invention to provide a new and improved electromagnetic transducer that can function as either a speaker, a microphone, or a control device.
It is still a further object of this invention to provide a new and improved electroacoustic transducer that produces increased power output at the high frequency range.
It is also an object of this invention to provide a new and improved electroacoustic transducer that is relatively light weight and can produce a wide range of frequencies, including the high, intermediate and low frequency ranges with a single diaphragm.
It is another object of the present invention to provide a new and improved electroacoustic transducer that is omidirectional in its sound propagation.